The present invention relates to semiconductor optical amplifiers.
The efficient transfer of optical energy is a primary design goal of semiconductor optical amplifiers (SOAs). Providing inexpensive fabrication methods is another. However, fabrication methods should be robust, allowing for variations to occur without compromising function and reliability. Polarization insensitive operation is another goal of SOA design. Polarization insensitivity is needed to provide consistent amplification of the optical signal, despite variations that occur in its polarization.
Up to the present time, the pursuit of such goals has been fraught with problems. Good signal coupling from the optical transmission fiber into the active region of the optical amplifier is needed for efficient energy transfer. However, signal coupling loss between the optical fiber and the SOA has been high in prior art SOAs, typically ranging between 5 and 7 dB or even more.
Some prior art processing methods, termed “material regrowth”, are intended to improve energy coupling efficiency of SOAs. However, such methods are difficult and costly to perform. In addition, they fail to achieve desired coupling efficiency. In a material regrowth process, the epitaxial layers of the substrate are not grown in a continuous sequence of steps. Instead, the substrate is processed incrementally in a deposition chamber to grow one or more layers, and then removed to a second chamber to perform one or more etch steps. After etching, the substrate is returned to a deposition chamber again for to grow more layers. Later, the substrate may be returned again to the second chamber to be etched.
Some less costly, easier to manufacture alternatives also fail to achieve good coupling efficiency between the optical transmission fiber and the active region of the SOA. Nor have such alternatives been able to provide polarization insensitive operation.
It would be desirable to provide a semiconductor optical amplifier (SOA) that is polarization insensitive and has high energy coupling efficiency between the optical transmission fiber and the SOA and/or between the SOA and an optical transmission fiber. It would further be desirable to provide a method of making an SOA that does not require the growth or regrowth of one or more epitaxial semiconductor layers of the SOA following a step of etching one or more epitaxial semiconductor layers.